The objective of this investigation is to provide morphological information on the action of metabolic hormones on rat liver. In the proposed work, emphasis is placed on hormonal stimulation of smooth endoplasmic reticulum and the role of this organelle in glycogen deposition. Three types of experiments are proposed: (1) Effects of corticosterone, in physiological doses, on SER and glycogen in fasted, adrenalectomized rats. (2) Action of insulin on SER and glycogen deposition in diabetic rats. (3) Effects of hormonal stimulation of isolated hepatocytes with respect to proliferation of SER and glycogen deposition. In these three experiments a search for the earliest morphological response to hormonal administration will be made.